They give birth to young ones.
Explanation
Animals are categorized based on the site of embryonic development and whether they lay eggs or give birth to young ones. This classification divides animals into two primary groups: oviparous and viviparous.
Option Analysis:
- They lay eggs with hard shells. is incorrect: Animals that lay eggs are known as oviparous. In terrestrial oviparous animals, such as reptiles and birds, the eggs are typically covered by hard calcareous shells for protection.
- Development of the embryo takes place outside the female body. is incorrect: In viviparous animals, the development of the embryo takes place inside the female body. Development outside the female body is characteristic of oviparous animals.
- They give birth to young ones. is correct: Viviparous animals are defined by the physiological process where the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered (born) directly from the female body. This group includes the majority of mammals, including humans.
- They undergo external fertilisation. is incorrect: Viviparous animals undergo internal fertilization, which is necessary for the embryo to develop inside the mother. External fertilization is generally associated with aquatic oviparous animals (e.g., amphibians and fishes), where gametes fuse in the external water medium.
Key Takeaway:
Viviparous animals are characterized by giving birth to live young ones following internal embryonic development, whereas oviparous animals lay eggs.